This invention relates to a pallet carrying vehicle system for interchanging pallets, to which workpieces are fixtured, between different machine tools and other pallet handling stations.
Pallet shuttle systems for interchanging workpiece carrying pallets between different machine tools and set-up stations in a manufacturing facility have incorporated various vehicles movable along trackways. Generally, a pair of tracks, which may be elevated or floor mounted, extend adjacent the series of machine tools and set-up stations. A vehicle car is driven by an electric motor which rotates wheels which frictionally move the car along the tracks to a particular station. A pair of rails, transverse with respect to the tracks, are slidably mounted on the car for extension outwardly to shuttle a pallet carried thereon to an adjacent station.
The pallet shuttle rails on such vehicles have carried a single pallet because it is advantageous to minimize the width of the vehicle in order to space the machine tools closer to the shuttle track. As a result, the vehicle must first travel to a machine tool and remove the pallet located thereon, transport the pallet to a different position, and then make a return trip with a new pallet which is to be received by the machine tool. To prevent machine tools from being idle while pallets are being shuttled, each machine tool is equipped with intermediate pallet storage or loading equipment so that a new pallet is immediately available to the machine tool upon removal of the old pallet.
To eliminate the necessity for intermediate pallet storage equipment at each machine tool, it has been known to mount a pair of side-by-side rail assemblies on a vehicle car. One traverse rail assembly is empty while the remaining rail assembly carries a new pallet. The vehicle car first aligns the empty rail assembly with the machine tool pallet changing mechanism, extends the rails, and withdraws the old pallet. The vehicle car is then driven to align the other pallet carrying rails with the machine tool pallet changer mechanism, and the new pallet is transferred to the machine tool. Unfortunately, this system requires vehicle movement during a pallet interchange, and requires that the machine tool be idle while the vehicle car moves the pair of rail assemblies in alignment with the pallet changer mechanism of the machine tool.
The pallet carrying rails of a vehicle car must be precisely positioned with respect to the pallet changer of the machine tool in order to effect interchange of a pallet. Often this positioning must be accurate within a few thousands of an inch. Such accuracy cannot be obtained by an electric motor which drives wheels frictionally resting on a track, due to slippage between the wheels and the track. A supplemental aligning system, such as a latching mechanism which captures or detents the vehicle at a precise position, has been required to finally align the pallet carrying rails with the pallet changer mechanism of the machine tool. The necessity for a separate vehicle latching mechanism at each machine tool unduly increases the complexity and costs of the shuttle system. Also, it is difficult to readily add new stations to an existing shuttle system.
Floor mounted pallet changers have been used with individual machine tools for removing an old pallet on a machine tool and depositing a new pallet. The pallet changers includes a pair of pallet carrying rails which extend and retract to retrieve a pallet, after which a platform rotates the rails by 180.degree., and then the rails extend to deposit a new pallet carried at the opposite end of the rails. Such pallet changers have been considered too long for advantageous use in a vehicle shuttle system. Furthermore, such pallet changers typically use hydraulic motors for rotational indexing and rail extension, and their hydraulically controlled mechanisms are not readily adaptable to be driven by electric motors of the type heretofore used on vehicle cars. As a result, such pallet changes have been limited to floor mounted units utilized with individual machine tools.